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Tuesday, 13 November 2012

Green manure

It is my firm belief that plants are good for the soil. Most of the garden should be a year-round living biomass of vegetation. The place of green manure is to fill temporary bare gaps. These often occur in the winter vegetable garden and green manuring is achieved by broadcasting seed of quick growing plants.

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Reasons to green manure

Organic matter is created by photosynthesizing plants.

The action of plant roots enhances soil structure.

Plant roots absorb nutrients that otherwise would be leached. Most nutrients are held naturally by the soil, but significantly there is no soil mechanism to hang on to nitrates. Sandy soils may also lose a little potassium. Note that leaching loss is a winter phenomenon. In most of the UK, summer rain will take nutrients deeper but they will not be lost to plant roots that follow them down.

Certain plants, classically legumes such as clovers, vetches, lupins and tares, have a symbiotic relationship with nitrogen fixing bacteria. These are present in clearly visible root nodules and are able to capture otherwise unavailable nitrogen gas from the air.

A dense sowing of green manure will help to suppress weeds. Personally, I have a zero tolerance of seeding weeds and such ground cover would impede my weeding.

In winter I am relaxed about weed cover but refuse to let weeds seed. This eclectic mix of self sown garden plants is hardly conventional green manure but the variegated clover will earn its keep!

Incorporating green manure into the soil

Some gardeners might have a problem
incorporating green manure at the end of it’s season. Not for me as a user of
glyphosate. When I anticipate needing space for sowing and planting I
glyphosate spray. The tops of the vegetation crumbles and enters the soil.
Green manure roots remain and decay in the soil. Undisturbed they will perhaps
provide a better starter substrate for new mycorrhizal growth that will support
the next crop

Soft vegetation
that decays on the surface is better for the soil structure than that composted
elsewhere and later returned. No matter, this difference at best is quite small and
many gardeners will choose to compost untidy dead green manure!

There is some evidence that soft vegetation rotavated into the ground ‘activates’ soil bacteria, whose ‘second course’ is the permanent soil organic matter we ought to preserve.

Those gardeners who do not use glyphosate might just cut back the tops of green manure and leave it to lie on the surface to decay. Perhaps hoe lightly in dry weather to detach the plants from their roots. If you are worried about slugs read thisGardeners with different methods to me might just dig it in!

These vegetables will remain overwinter. They are my favorite green manure!

Studies have shown that levels of glomalin appear to be raised by cover crops/green manures, reduced phosphorus input, and very minimal use of crops that don’t have arbuscular mycorrhizal fungi on their roots. Those include members of the Brassicaceae and mustard families. Growing these crops is similar to leaving the ground fallow because glomalin production stops. You need to rotate them with crops that have glomalin-producing fungi.

Thanks for this Sonja, I had rather overlooked the implications of glomalin in my post. I Have recently been reading about some very interesting research on restoring soil structure by sowing tall fescue grass inoculated with arbuscular fungi. I am aware that brassicas are among the 10% of higher plants that do not usually form mycorrhizal associations. I have some difficulty with the implications of this. I remember many years ago I attended a vegetable growers conference where the speaker was a Government senior soil scientist. A question from the floor was “We know one of the best ways of restoring soil structure is to grass down, which of the vegetables we grow most improves structure?” The answer flashed back,“brassicas”. (This was long before Sara Wright discovered glomalin in 1996). Ever since I have been impressed by how the deep and spreading roots of brassicas do seem to have improved my soil and you will see from my picture, sprouts and sprouting broccoli are very prominent in my winter garden.

I suspect rotation is the answer Roger. Despite inhibiting glomalin production, brassicaceae do indeed restore the structure of soil, especially no-dig soils, because of their deep roots. One of the very best cover crops for this is the Daikon radish, which I think has already been mentioned in the comments of a previous post.

That was me Sonja. I love Daikon radishes – also called mooli. They are a superb soil buster in a no-dig garden and are both tasty and healthy to eat. I first came across them in Japan and Korea, but have since found out that they are also a big part of the cuisine in India and Pakistan.

Phacelia (scorpionweed) is one of the top 20 nectar-producing flowers for honeybees and is also very attractive to bumblebees and hoverflies. Hoverflies are a voracious predator of aphids so there are multiple benefits!

Couldn’t agree more Helen! Phacelia is brilliant - for bees and for soil! It germinates at low temperatures and is tolerant of cold so is ideal for sowing from March onwards, giving an early source of nectar to bees.